Segregation-based memory

ABSTRACT

Methods, systems, and devices for operating memory cell(s) are described. A resistance of a storage element included in a memory cell may be programmed by applying a voltage to the memory cell that causes ion movement within the storage element, where the storage element remains in a single phase and has different resistivity based on a location of the ions within the storage element. In some cases, multiple of such storage elements may be included in a memory cell, where ions within the storage elements respond differently to electric pulses, and a non-binary logic value may be stored in the memory cell by applying a series of voltages or currents to the memory cell.

CROSS REFERENCE

The present Application for Patent is a divisional of U.S. patentapplication Ser. No. 16/102,493 by Boniardi et al., entitled“SEGREGATION-BASED MEMORY,” filed Aug. 13, 2018, assigned to theassignee hereof, and is expressly incorporated by reference in itsentirety herein.

BACKGROUND

The following relates generally to operating a memory system and morespecifically to segregation-based memory (SBM).

Memory devices are widely used to store information in variouselectronic devices such as computers, wireless communication devices,cameras, digital displays, and the like. Information is stored byprograming different states of a memory device. For example, binarydevices have two states, often denoted by a logic “1” or a logic “0.” Inother systems, more than two states may be stored. To access the storedinformation, a component of the electronic device may read, or sense,the stored state in the memory device. To store information, a componentof the electronic device may write, or program, the state in the memorydevice.

Various types of memory devices exist, including magnetic hard disks,random access memory (RAM), read-only memory (ROM), dynamic RAM (DRAM),synchronous dynamic RAM (SDRAM), ferroelectric RAM (FeRAM), magnetic RAM(MRAM), resistive RAM (RRAM), flash memory, phase change memory (PCM),and others. Memory devices may be volatile or non-volatile. Non-volatilememory (e.g., FeRAM, PCM, RRAM) may maintain their stored logic statefor extended periods of time even in the absence of an external powersource. Volatile memory devices (e.g., DRAM) may lose their stored stateover time unless they are periodically refreshed by an external powersource.

PCM may store different logic values using materials that exhibitdifferent resistances at different material phases—e.g., an amorphousphase may correspond to a first logic value and a crystalline phase maycorrespond to a second logic value. The phase of a material may also bereferred to as a state of the material—e.g., an amorphous state orcrystalline state. In some cases, due to timing or other constraints ofwide bandgap materials, PCM may use narrow bandgap materials instead ofwide bandgap materials, despite the smaller memory windows that narrowbandgap materials may provide.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an example of a memory array that supports theoperation of segregation-based memory (SBM) in accordance with variousaspects of the present disclosure.

FIG. 2A illustrates an example of a memory array that supports SBM inaccordance with various aspects of the present disclosure.

FIG. 2B illustrates exemplary diagrams for the operation of SBM inaccordance with various aspects of the present disclosure.

FIG. 3 illustrates an example of a memory array that supports SBM andexemplary voltage plots for the operation of SBM in accordance withvarious aspects of the present disclosure.

FIG. 4 illustrates a block diagram of a memory array that supports theoperation of SBM in accordance with various embodiments of the presentdisclosure.

FIG. 5 illustrates a system, including a memory array, that supports theoperation of SBM in accordance with various embodiments of the presentdisclosure.

FIGS. 6 and 7 illustrate flowcharts of a method or methods for theoperation of SBM in accordance with various embodiments of the presentdisclosure.

DETAILED DESCRIPTION

Electronic data may be stored in memory cells containing storageelements that are electronically programmable (e.g., by way of applyinga voltage or current) to have different resistivity (e.g., a resistanceor threshold voltage). Such memory may be referred to as resistivememory. For certain types of resistive memory, a logic value may bestored in a memory cell by changing a resistance of a storage element ofthe memory cell, where a first resistance of the storage elementcorresponds to a first logic value and a second resistance of thestorage element corresponds to a second logic value. Additionally oralternatively, a logic value may similarly be stored by changing athreshold voltage of the storage element (which in some cases may relateto an observed resistivity of the storage element).

A storage element may be composed of a material having an electronicallyprogrammable resistance. Certain materials may have differentresistivity based on an electronically programmable phase (or state) ofthe material. For example, certain materials may exhibit differentresistivity based on a phase of the material—e.g., based on whether thematerial is in an amorphous phase or a crystalline phase. Such materialsmay be referred to as phase change materials. In some examples, a phaseof a phase change material may be changed by applying one or morevoltages across (or currents to—e.g., through) the phase changematerial, heating the material to at least a certain temperature (whichmay be referred to as the glass transition temperature) and cooling thematerial (which may also be referred to as quenching) by removing oraltering the applied voltage or current so as to cause a phase of thephase change material to transition from one phase to another phase.

In some cases, the glass transition temperature for a particular phasechange material may be based on the chemical structure of the phasechange material (e.g., certain phase change materials may have higher orlower glass transition temperatures than other phase change materials).The time for transitioning from the amorphous to the crystalline phasemay be referred to as the “SET period,” and the time for transitioningfrom the crystalline phase to the amorphous phase may be referred to asthe “RESET period.” In some cases, the duration of a SET period is basedon a bandgap and/or glass transition temperature of a phase changematerial—e.g., the duration of the SET period may increase with thebandgap and glass transition temperature. Chalcogenide materials may beexamples of phase change materials.

Some materials—in addition or in the alternative to being phase changematerials—may exhibit different resistivity based on how ions aregrouped within the material—e.g., based on whether a majority of ionswithin the material are at one portion (e.g., in a segregated state) oranother portion (e.g., in a migrated state) of the material. Suchmaterials may be referred to as segregable materials.

A change in the resistivity of a segregable material may occur without aphase of the segregable material being changed. In some examples, alocation of the ions within a segregable material may be changed byapplying voltages across (or currents to) the segregable material,causing the ions to move (e.g., segregate/migrate) to a particular endof the segregable material. The time for transitioning between differentsegregated states may be referred to as the “segregation period.”

In some cases, whether a segregable material experiences ion segregationin response to an applied voltage or current is based on the chemicalstructure of the segregable material—e.g., certain segregable materialsmay respond to higher or lower voltages or current than other segregablematerials. Ion segregation may occur in response to shorter, lowerintensity electric pulses relative to phase change—e.g., since meltingof the material does not occur. In some cases, a material may be both aphase change material and a segregable material, and applied electricpulses may be configured to operate the material as either a phasechange material by changing the phase of the material or as a segregablematerial by segregating ions within the material without inducing aphase change. For example, chalcogenide materials may also be examplesof segregable materials.

A logic value stored in resistive memory may be determined by sensing aresistivity of a storage element contained in a memory cell. Among othertechniques, the resistivity of the storage element may be sensed byapplying a voltage to a memory cell and comparing a resulting currentwith expected currents for possible logic values stored at the memorycell, where the expected currents are determined based on expectedresistivity of the storage element for different logic values. In suchcases, the larger the difference in resistivity of the storage elementbetween different logic states, the more reliably (e.g., with lowererror rates) the logic states may be determined—e.g., because thedifference between the measured current and expected currents can begreater. In some examples, the difference between the resistivity ofdifferent logic states corresponds to the memory window of a memorycell, and a larger memory window may be desirable.

The bandgap energy of a material may be correlated with a resultingmemory window for a memory cell. For instance, for phase change memory(PCM)—a memory system that programs memory cells by changing a phase ofthe storage elements—phase change materials with wider bandgapstypically provide larger memory windows than phase change materials withnarrow bandgaps. But, the SET period for wider bandgap phase changematerials is typically longer than the SET period for smaller bandgapphase change materials—e.g., due to higher glass transition temperaturesof the wider bandgap phase change materials relative to smaller bandgapphase change materials. In some cases, a duration of the SET period fora wide bandgap phase change material may exceed timing constraints, anda narrower bandgap phase change material may be used instead of the widebandgap phase change material, despite the narrower bandgap phase changematerial providing a smaller memory window.

Similar to phase change materials, segregable materials with widerbandgaps may provide larger memory windows than segregable materialswith narrower bandgaps. In some examples, a segregation period for awide bandgap segregable material is shorter than a SET period for asimilarly wide bandgap phase change material. In some examples, asegregation period for a material that is both segregable and capable ofphase change may satisfy timing constraints that are not satisfied by aSET period for the same material.

In some cases, data may be stored in a memory array using ionsegregation techniques instead of phase change techniques—memory that isprogrammed by inducing ion movement within a storage element may bereferred to as segregation-based memory (SBM). This way, larger bandgapmaterials having short segregation periods may be used in a memorydevice to provide larger memory windows without violating timingconstraints.

Also, certain phase change materials having shorter segregation periodsthan SET periods may be operated using ion segregation techniques,reducing the latency of a memory device using such materials. Also,since SBM programs storage elements within a single phase (e.g., byraising a temperature of a storage element above its glass transitiontemperature but without melting the storage element), lower temperaturesmay be generated during programming (e.g., due to using low intensitycurrent pulses), reducing thermal disturbance in a memory device. Also,as discussed above, relatively short electric pulses may be used toinduce ion segregation in a material, reducing latency of a memorydevice using SBM.

For example, a segregable material (e.g., a chalcogenide material) maybe included in a memory cell. To program a resistance of the material, avoltage that causes the ions within the material to move to one portion(e.g., the top or bottom) of the material without changing a phase ofthe material (e.g., from an amorphous to a crystalline phase, or viceversa) may be applied to the memory cell. That is, a magnitude andduration of the applied voltage that causes ion segregation within amaterial while heating the material to a temperature that is above aglass transition temperature and below a melting temperature for thatmaterial may be used. The magnitude and duration of the applied voltagemay also be configured so that, for an applied voltage, the segregationperiod is shorter than a SET period for the material.

The different segregated states of the material may correspond todifferent logic values. For instance, a resistance corresponding to theions moving to a first portion (e.g., the top) of the material may beassociated with a first logic value and a resistance corresponding tothe ions moving to a second portion (e.g., the bottom) of the materialmay be associated with a second logic value. This way, a segregablematerial may be operated to store different logic states withoutundergoing a phase transition.

As discussed above, the resistivity of different materials may beprogrammable with different voltages or currents. For example, ions inone segregable material may move in response to an applied voltage (orcurrent) while ions in another segregable material may not move inresponse to the same applied voltage (or current).

In some cases, multiple materials that are independently programmablemay be included in and used as storage elements in a memory cell. Thisway, the materials may be individually programmed by applying differentelectric pulses and non-binary data may be stored in a memory array,increasing a memory-density of a corresponding memory device—e.g., thememory device may store more data with little to no increase in thefootprint of the memory device.

For example, multiple segregable materials may be included as storageelements in a memory cell. In some cases, a segregable material mayfunction as both a storage element and a selection device for the memorycell. In some examples, one of the segregable materials may have a widebandgap and a high glass transition temperature while another of thesegregable materials alloy may have a narrow bandgap and low glasstransition temperature. In some examples, one of the segregablematerials is in an amorphous phase and another segregable material is ina crystalline phase. In some examples, each of the segregable materialsmay be in an amorphous phase. In some examples, each of the segregablematerials may be chalcogenide materials.

For example, in a memory cell comprising two segregable materials, aseries of voltages or currents may be applied to the memory cell toprogram the resistivity of the segregable materials.

For example, a first voltage in the series of voltages having a firstmagnitude (e.g., a relatively high intensity voltage of one polarity,resulting in a relatively high current in one direction) and firstduration may be applied to the memory cell, and ions in both of thesegregable materials may move to one portion (e.g., the top) of therespective segregable materials. And a second voltage in the series ofvoltages having a second magnitude (e.g., a relatively low intensityvoltage of another polarity, resulting in a relatively low current inanother direction) and second duration may be applied to the memorycell, and ions in only one of the segregable materials may move toanother portion (e.g., the bottom) of the respective segregablematerial. In some cases, the first and second duration may be the same.During and after the application of the first and second voltages, bothof the segregable materials may remain in the respective state (phase)they were in before the first and second voltages were applied.

The different combinations of segregated states of the segregablematerials may correspond to different resistivity combinations, whichmay correspond to different logic values. In some cases, the differentcombination of segregated states may correspond to more than two logicvalues. For example, a resistivity of one segregable materialcorresponding to the ions moving to a portion (e.g., the top) of thesegregable material and a resistivity of the other segregable materialcorresponding to the ions moving to a portion (e.g., the bottom) of theother segregable material may correspond to a first logic state. Aresistance of one segregable material corresponding to the ions movingto a portion (e.g., the top) of the segregable material and a resistanceof the other segregable material corresponding to the ions moving to aportion (e.g., the top) of the other segregable material may correspondto a second logic state.

A resistance of one segregable material corresponding to the ions movingto a portion (e.g., the bottom) of the segregable material and aresistance of the other segregable material corresponding to the ionsmoving to a portion (e.g., the top) of the other segregable material maycorrespond to a third logic state, and so on. This way, multiple bits ofinformation may be stored by a memory cell by applying particularvoltage sequences to the memory cell, increasing the density of a memoryarray.

Features of the disclosure introduced above are further described belowin the context of a memory system. Specific examples of operating amemory system using SBM are then described. These and other features ofthe disclosure are further illustrated by and described with referenceto apparatus diagrams, system diagrams, and flowcharts that relate toSBM.

FIG. 1 illustrates an example of a memory array that supports theoperation of SBM in accordance with various aspects of the presentdisclosure. Memory array 100 may be an example of a memory system andmay include memory cells 105 that are programmable to store differentstates. Each memory cell 105 may be programmable to store two states,denoted a logic 0 and a logic 1. In some cases, each memory cell 105 isconfigured to store more than two logic states.

A memory cell 105 may include a memory element having programmablestates. For example, a memory cell 105 in a DRAM or FeRAM architecturemay store a charge representative of the programmable states in acapacitor; for example, in DRAM a charged and uncharged capacitor mayrepresent two logic states, respectively. DRAM architectures may use acapacitor including a dielectric material and having linear electricpolarization properties as a memory element. FeRAM architectures may usea similar design as DRAM, but the capacitor used as the memory elementmay include a ferroelectric material with non-linear polarizationproperties

As another example, a memory cell 105 in a PCM architecture may change aresistivity (e.g., resistance or threshold voltage) of a memory elementby changing the phase (state) of the memory element, where a differentresistivity may represent a different logic state. Thus, PCMarchitectures may use, as a memory element, a material having a variableresistance as a function of the phase (state) of the memory element. Forexample, a material configurable to have a crystalline phase and anamorphous phase with different respective electrical resistances may beused as a memory element. A voltage applied to the memory cell 105 mayresult in different currents depending on whether the material is in acrystalline or an amorphous phase, and the magnitude of the resultingcurrent may be used to determine the logic state stored by memory cell105. In some cases, the memory cell 105 may have a combination ofcrystalline and amorphous areas that may result in intermediateresistances, which may correspond to different logic states (e.g.,states other than logic 1 or logic 0) and may allow memory cells 105 tostore more than two different logic states. As discussed below, thelogic state of a memory cell 105 may be configured in part by heating,including melting, the memory element. In some examples, PCMarchitectures may use chalcogenide materials as memory elements.

A memory cell 105 in an SBM architecture in accordance with theteachings herein may, however, change a resistivity of a memory elementwithout changing the phase of the memory element by changing thelocation of ions within the memory element, where a differentresistivity may represent a different logic state. For example, SBMarchitectures may use, as a memory element, a material having a variableresistance as a function of ion location in the material. The segregablematerial in an SBM memory cell may have different resistances within asingle phase—e.g., may have a variable resistance while remaining withinan amorphous or crystalline phase. In some cases, the memory cell 105may include multiple segregable materials which may be programmableusing different voltages or currents. In some examples, the differentmaterials may be programmed into different combinations of segregatedstates, allowing memory cells 105 to store more than one logic state. Asdiscussed below, the logic state of a memory cell 105 may be set byapplying a voltage or series of voltages to a memory cell in asegregation-based architecture. In some examples, segregation-basedarchitectures may use chalcogenide materials as memory elements. In someexamples, a material may be capable of being included in either a PCM oran SBM architecture. That is, a resistance of such a material may beprogrammable by changing a phase of the material and/or a location ofions within the material.

Memory array 100 may be a three-dimensional (3D) memory array, wheretwo-dimensional (2D) memory arrays are formed on top of one another.This may increase the number of memory cells that may formed on a singledie or substrate as compared with 2D arrays, which in turn may reduceproduction costs or increase the performance of the memory array, orboth. According to the example depicted in FIG. 1 , memory array 100includes two levels of memory cells 105 and may thus be considered athree-dimensional memory array; however, the number of levels is notlimited to two. Each level may be aligned or positioned so that memorycells 105 may be approximately aligned with one another across eachlevel, forming a memory cell stack 145.

Each row of memory cells 105 is connected to an access line 110, andeach column of memory cells 105 is connected to a bit line 115. Accesslines 110 may also be known as word lines 110, and bit lines 115 mayalso be known digit lines 115. References to word lines and bit lines,or their analogues, are interchangeable without loss of understanding oroperation. Word lines 110 and bit lines 115 may be substantiallyperpendicular to one another to create an array. As shown in FIG. 1 ,the two memory cells 105 in a memory cell stack 145 may share a commonconductive line such as a bit line 115. That is, a bit line 115 may bein electronic communication with the bottom electrode of the uppermemory cell 105 and the top electrode of the lower memory cell 105.Other configurations may be possible, for example, a third layer mayshare a word line 110 with a lower layer. In general, one memory cell105 may be located at the intersection of two conductive lines such as aword line 110 and a bit line 115. This intersection may be referred toas a memory cell's address. A target memory cell 105 may be a memorycell 105 located at the intersection of an energized word line 110 andbit line 115; that is, a word line 110 and bit line 115 may be energizedin order to read or write a memory cell 105 at their intersection. Othermemory cells 105 that are in electronic communication with (e.g.,connected to) the same word line 110 or bit line 115 may be referred toas untargeted memory cells 105.

As discussed above, electrodes may be coupled to a memory cell 105 and aword line 110 or a bit line 115. The term electrode may refer to anelectrical conductor, and in some cases, may be employed as anelectrical contact to a memory cell 105. An electrode may include atrace, wire, conductive line, conductive layer, or the like thatprovides a conductive path between elements or components of memoryarray 100.

Operations such as reading and writing may be performed on memory cells105 by activating or selecting a word line 110 and bit line 115, whichmay include applying a voltage or a current to the respective line. Wordlines 110 and bit lines 115 may be made of conductive materials, such asmetals (e.g., copper (Cu), aluminum (Al), gold (Au), tungsten (W),titanium (Ti), etc.), metal alloys, carbon, or other conductivematerials, alloys, or compounds. Upon selecting a memory cell 105, theresulting signal may be used to determine the stored logic state. Forexample, a voltage may be applied and the resulting current may be usedto differentiate between the resistive states of the phase changematerial.

Accessing memory cells 105 may be controlled through a row decoder 120and a column decoder 130. For example, a row decoder 120 may receive arow address from the memory controller 140 and activate the appropriateword line 110 based on the received row address. Similarly, a columndecoder 130 receives a column address from the memory controller 140 andactivates the appropriate bit line 115. Thus, by activating a word line110 and a bit line 115, a memory cell 105 may be accessed.

Upon accessing, a memory cell 105 may be read, or sensed, by sensecomponent 125. For example, sense component 125 may be configured todetermine the stored logic state of memory cell 105 based on a signalgenerated by accessing memory cell 105. The signal may include a voltageor electrical current, and sense component 125 may include voltage senseamplifiers, current sense amplifiers, or both. For example, a voltagemay be applied to a memory cell 105 (using the corresponding word line110 and bit line 115) and the magnitude of the resulting current maydepend on the electrical resistance of the memory cell 105. Likewise, acurrent may be applied to a memory cell 105 and the magnitude of thevoltage to create the current may depend on the electrical resistance ofthe memory cell 105. Sense component 125 may include various transistorsor amplifiers in order to detect and amplify a signal, which may bereferred to as latching. The detected logic state of memory cell 105 maythen be output as output 135. In some cases, sense component 125 may bea part of column decoder 130 or row decoder 120. Or, sense component 125may be connected to or in electronic communication with column decoder130 or row decoder 120.

A memory cell 105 may be set, or written, by similarly activating therelevant word line 110 and bit line 115—e.g., a logic value may bestored in the memory cell 105. Column decoder 130 or row decoder 120 mayaccept data, for example input 135, to be written to the memory cells105. In the case of PCM, a memory cell 105 is written by heating thememory element, for example, by passing a current through the memoryelement. This process is discussed in more detail below. In the case ofSBM, a memory cell 105 is written by passing a current through thememory element while refraining from heating the memory element to aglass-transition temperature. In some cases, SBM may include memorycells 105 having multiple storage elements which may be individuallyprogrammed by applying voltages (or currents) of different magnitudesand/or durations to a memory cell, yielding different combinations ofresistivity across storage elements and thus different collectiveresistivities of the memory cell.

In some memory architectures, accessing the memory cell 105 may degradeor destroy the stored logic state and re-write or refresh operations maybe performed to return the original logic state to memory cell 105. InDRAM, for example, a logic-storing capacitor may be partially orcompletely discharged during a sense operation, corrupting the storedlogic state. So the logic state may be re-written after a senseoperation. Additionally, activating a single word line 110 may result inthe discharge of all memory cells in the row; thus, all memory cells 105in the row may need to be re-written. But in non-volatile memory, suchas PCM and SBM, accessing the memory cell 105 may not destroy the logicstate and, thus, the memory cell 105 may not require re-writing afteraccessing.

Some memory architectures, including DRAM, may lose their stored stateover time unless they are periodically refreshed by an external powersource. For example, a charged capacitor may become discharged over timethrough leakage currents, resulting in the loss of the storedinformation. The refresh rate of these so-called volatile memory devicesmay be relatively high, e.g., tens of refresh operations per second forDRAM, which may result in significant power consumption. Withincreasingly larger memory arrays, increased power consumption mayinhibit the deployment or operation of memory arrays (e.g., powersupplies, heat generation, material limits, etc.), especially for mobiledevices that rely on a finite power source, such as a battery. Asdiscussed herein, non-volatile PCM and SBM cells may have beneficialproperties. For example, PCM and SBM may offer comparable read/writespeeds as DRAM but may be non-volatile and allow for increased celldensity.

The memory controller 140 may control the operation (read, write,re-write, refresh, etc.) of memory cells 105 through the variouscomponents, for example, row decoder 120, column decoder 130, and sensecomponent 125. In some cases, one or more of the row decoder 120, columndecoder 130, and sense component 125 may be co-located with the memorycontroller 140. Memory controller 140 may generate row and columnaddress signals in order to activate the desired word line 110 and bitline 115. Memory controller 140 may also generate and control variousvoltage potentials or currents used during the operation of memory array100. In general, the amplitude, shape, or duration of an applied voltageor current discussed herein may be adjusted or varied and may bedifferent for the various operations discussed in operating memory array100. Furthermore, one, multiple, or all memory cells 105 within memoryarray 100 may be accessed simultaneously; for example, multiple or allcells of memory array 100 may be accessed simultaneously during a resetoperation in which all memory cells 105, or a group of memory cells 105,are set to a single logic state.

In some examples, the memory controller 140 may select memory cell 105for a write operation, where the memory cell includes a first materialin a first phase (e.g., an amorphous phase) having a first resistivitythat corresponds to a first logic value stored by the first material.The memory controller 140 may also apply a first voltage to memory cell105 during the write operation. In some cases, the first material mayremain in the first phase and have a second resistivity that correspondsto a second logic value stored by the first material after the firstvoltage is applied.

In some examples, the memory controller 140 may select memory cell 105for a write operation, where the memory cell may include a firstmaterial having a first resistivity and a second material having asecond resistivity. The memory controller 140 may apply a first voltageto memory cell 105 during the write operation, where the first materialhas a third resistivity and the second material has a fourth resistivityafter the first voltage is applied. The memory controller 140 may alsoapply a second voltage to memory cell 105 during the write operation,where the first material has the first resistivity and the secondmaterial having the fourth resistivity after the second voltage isapplied. In some cases, the first material may remain in the first phase(e.g., an amorphous phase) before and after the first and secondvoltages are applied and the second material may remain in the secondphase (e.g., a crystalline phase or an amorphous phase) before and afterthe first and second voltages are applied.

FIG. 2A illustrates an example of a memory array that supports SBM inaccordance with various aspects of the present disclosure. Memory array200 may be an example of a memory array or a representative portionthereof, and may include memory cell 105-a, word line 110-a, and bitline 115-a, which may be examples of a memory cell 105, word line 110,and bit line, as discussed with reference to FIG. 1 .

The structure of memory array 200 may be referred to as a cross-pointarchitecture. It may also be referred to as a pillar structure. Forexample, as shown in FIG. 2 , a pillar may be in contact with a firstconductive line (word line 110-a) and a second conductive line (bit line115-a). Such a pillar architecture may offer high-density data storagewith low production costs. For example, the cross-point architecture mayhave memory cells with a reduced area and, resultantly, an increasedmemory cell density compared to at last some other architectures. Insome examples, the architecture may have a 4F² memory cell area, where Fis the smallest feature size, compared to other architectures with a 6F²memory cell area, such as those with a three-terminal selection.

Memory array 200 may be made by various combinations of materialformation and removal. For example, layers of material may be depositedor otherwise formed that correspond to word line 110-a, bottom electrode225, second memory element 220, middle electrode 215, first memoryelement 210, and top electrode 205. Material may be selectively removedto then create the desired features, such as the pillar structuredepicted in FIG. 2 . For example, features may be defined usingphotolithography to pattern a photoresist and then material may beremoved by techniques such as etching. Bit lines 115-a may then beformed, for example, by depositing a layer of material and selectivelyetching to form the line structure depicted in FIG. 2 . In some cases,electrically insulating regions or layers may be formed or deposited.The electrically insulating regions may include oxide or nitridematerials, such as silicon oxide, silicon nitride, or other electricallyinsulating materials.

Various techniques may be used to form materials or components of memoryarray 200. These may include, for example, chemical vapor deposition(CVD), metal-organic chemical vapor deposition (MOCVD), physical vapordeposition (PVD), sputter deposition, atomic layer deposition (ALD), ormolecular beam epitaxy (MBE), among other thin film growth techniques.Material may be removed using a number of techniques, which may include,for example, chemical etching (also referred to as “wet etching”),plasma etching (also referred to as “dry etching”), orchemical-mechanical planarization.

In some cases, a 3D memory array may be formed by stacking multiplememory arrays 200 on one another. The two stacked memory arrays may, insome examples, have common conductive lines such that each level mayshare word lines 110 or bit lines 115 as described with reference toFIG. 1 .

Memory cell 105-a includes top electrode 205, first memory element 210,middle electrode 215, second memory element 220, which may be achalcogenide material, and bottom electrode 225. Memory cell 105-a maybe a target memory cell—e.g. a memory cell that has been selected for amemory operation (e.g., a read operation or a write operation).

Second memory element 220 of memory cell 105-a may include a materialwith a variable resistance. Variable resistance materials may refer tovarious material systems, including, for example, metal oxides,chalcogenides, and the like. In some examples, second memory element 220is composed of a chalcogenide material comprising indium (In)—Sb—Te(IST) or In—Ge—Te (IGT). IST and IGT may have wide bandgaps and highglass transition temperatures (e.g., between 400 Kelvin (K) and 450 K)relative to a material included in first memory element 210.

In some examples, second memory element 220 may be programmable bychanging a phase of the material (e.g., from an amorphous phase to acrystalline phase). Changing the phase of second memory element 220 mayexploit a large resistance contrast between crystalline and amorphousphases of a material included in second memory element 220. Secondmemory element 220 may be changed from a first phase (e.g., amorphous)to another phase (e.g., crystalline) by heating second memory element220 to at least a glass transition temperature of a material therein.Second memory element 220 may be changed back to the first phase (e.g.,amorphous) by heating second memory element 220 to the meltingtemperature and by then quickly cooling second memory element 220.

In some cases, the period of time during which second memory element 220transitions from the amorphous to the crystalline phase is referred toas the SET period and the period of time during which second memoryelement 220 transitions from the crystalline to the amorphous isreferred to as the RESET period. For materials having wide bandgaps(e.g., IST or IGT), the duration of a corresponding SET period (e.g.,˜100 μs) may exceed a maximum allowed duration for a memory operation.Thus, materials having wide bandgaps may not be used in PCM despiteproviding larger memory windows.

In some examples, second memory element 220 may additionally oralternatively be programmable using ion segregation. Such segregablematerials exploit the large resistance contrast between a first andsecond state of ions within segregable materials, which may be metaloxides, chalcogenides, and the like. In some cases, the first and secondstate of ions within a material may be referred to as segregated ormigrated states of the material. A material in a first segregated statemay have a majority of ions located near one end of the material, whichmay result in a high electrical resistance and/or threshold voltage ofthe material. A material in a second segregated state may have amajority of ions located near another end of the material, which mayresult in a low electrical resistance and/or threshold voltage of thematerial.

In some examples, a first resistivity of second memory element 220 maycorrespond to a first segregated state of ions within second memoryelement 220 and a second resistivity of second memory element 220 maycorrespond to a second segregated state of ions within second memoryelement 220. In some examples, a resistivity of second memory element220 corresponds to (e.g., depends at least in part on) a thresholdvoltage of second memory element 220. In some cases, a thickness ofsecond memory element 220 when configured to be programmed using ionsegregation may be reduced relative to a thickness of second memoryelement 220 when configured to be programmed using phase changetechniques. For example, the thickness of second memory element 220 maybe reduced by up to 50% when configured to be operated using ionsegregation.

To set a low-resistivity state, a voltage or current having a firstpolarity (e.g., negative, which may refer to having the voltage of thebit line 115-a lower than the voltage of the word line 110-a) may beapplied to memory cell 105-a. Applying the voltage of the first polaritymay cause the ions within second memory element 220 to move to one endof second memory element 220. To set a high-resistivity state, a voltageor current having a second polarity (e.g., positive, which may refer tohaving the voltage of the bit line 115-a higher than the voltage of theword line 110-a) may be applied to memory cell 105-a. Applying thevoltage of the second polarity may cause the ions within second memoryelement 220 to move to another end of second memory element 220. In somecases, the period of time during which second memory element 220transitions from a first segregated state to a second segregated stateis referred to as the segregation period. For certain materials (e.g.,IST or IGT), a duration of the segregation period may be shorter than aduration of a SET period (e.g., ˜100 μs). Regardless of polarity, amagnitude and duration of the voltage may be configured to cause ionmovement within second memory element 220 while avoiding heating secondmemory element 220 to a melting temperature and within a minimum periodof time (e.g., a minimum read duration for a memory operation).

Although a negative polarity is generally described as being when avoltage of bit line 115-a is lower than a voltage of word line 110-a anda positive polarity as being when a voltage of bit line 115-a is higherthan a voltage of word line 110-a, these designations are merely labelsand may be swapped without change of operation so long as the magnitudeand polarity of the above voltages are maintained.

First memory element 210 of memory cell 105-a may also include amaterial with a variable resistance. In some cases, first memory element210 may be connected in series between second memory element 220 and aconductive line, for example, between memory cell 105-a and at least oneof word line 110-a or bit line 115-a. For example, first memory element210 may be located between bottom electrode 225 and middle electrode215; thus, first memory element 210 may be located in series betweensecond memory element 220 and word line 110-a. Other configurations arepossible. For example, first memory element 210 may be located in seriesbetween second memory element 220 and bit line 115-a (e.g., thepositions of first memory element 210 and second memory element 220 maybe swapped).

First memory element 210 may be an electrically non-linear component(e.g., a non-ohmic component) such as a metal-insulator-metal (MIM)junction, an ovonic threshold switch (OTS), or ametal-semiconductor-metal (MSM) switch, among other types oftwo-terminal select devices, such as a diode. In some cases, firstmemory element 210 is a chalcogenide film such as, for example, an alloycomprising Se, arsenic (As), and Ge (SAG). In some cases, first memoryelement 210 has a narrow bandgap and a low glass transition temperature(e.g., between 350 K and 400 K) relative to second memory element 220.In some examples, a thickness of first memory element 210 is around 20nm.

In some cases, first memory element 210 may be programmed using ionsegregation, individually or in combination with second memory element220, as discussed with reference to second memory element 220. In someexamples, a first resistivity of first memory element 210 may correspondto a first segregated state of ions within first memory element 210 anda second resistivity of first memory element 210 may correspond to asecond segregated state of ions within first memory element 210. In someexamples, a resistivity of first memory element 210 corresponds to athreshold voltage of first memory element 210.

In some examples, first memory element 210 may be used to store a firstbit of a logic value (e.g., the least significant bit (LSB)) and secondmemory element 220 may be used to store a second bit of the logic value(e.g., the most significant bit (MSB)). In some examples, ions in firstmemory element 210 respond to different voltages and/or currents (e.g.,voltages and/or currents with different magnitudes) than ions in secondmemory element 220. For example, a first current having a first durationmay cause ion movement in first memory element 210 but not in secondmemory element 220. And a second current having a second duration (e.g.,larger in magnitude than the first current) may cause ion movement inboth first memory element 210 and second memory element 220. In somecases, the first and second duration may be the same.

That is, a resistivity of first memory element 210 may be programmablewithin an initial phase (e.g., while remaining in an amorphous phase)using an amount of current that is larger than a first threshold and aresistivity of second memory element 220 may be programmable within aninitial phase (e.g., while remaining in an amorphous or crystallinephase) using an amount of current that is larger than a secondthreshold, where the first threshold may be lower than the secondthreshold. In some examples, a resistivity of both first memory element210 and second memory element 220 is programmable with an amount ofcurrent that is below a third threshold, where the third threshold isbased on an amount of current that causes either first memory element210 or second memory element 220 to reach a glass transitiontemperature.

In some examples, a bandgap energy of first memory element 210 is lowerthan a bandgap energy of second memory element 220. In some cases,intermediate logic states (e.g., 01 and 10) stored by the combination offirst memory element 210 and second memory element 220 may experiencelittle to no amorphization drift relative to phase change programming offirst memory element 210 and second memory element 220 because theinitial phase of first memory element 210 and second memory element 220remain the same throughout the programming process.

In some cases, first memory element 210 may alternatively oradditionally be used as a selection component. First memory element 210may aid in selecting a particular memory cell 105-a or may help preventstray currents from flowing through non-selected memory cells 105-aadjacent a selected memory cell 105-a. It may also reduce the biasacross untargeted memory cells. For example, first memory element 210may have a threshold voltage such that a current flows through firstmemory element 210 when the threshold voltage is met or exceeded.

For SBM architectures, one technique for sensing the logic state storedat second memory element 220 includes passing a current through memorycell 105-a and measuring a resulting voltage. Another technique forsensing the logic state stored at second memory element 220 may be toapply an increasing voltage across memory cell 105-a and to measure theresulting current.

Voltage plot 201 depicts voltages applied as a function of time duringan example access operation of a target memory cell, such as memory cell105-a, that includes applying an aggregate voltage across the targetmemory cell by applying opposite polarity voltages to both access lines.Prior to accessing memory cell 105-a, both the word line 110-a and bitline 115-a may be maintained at a first voltage (e.g., at virtualground). In some examples, to access memory cell 105-a, a voltage may beapplied to bit line 115-a while word line 110-a is maintained at avirtual ground.

Accordingly, cell access voltage 240 may be applied across memory cell105-a, where cell access voltage 240 may be equal to the voltage appliedto bit line 115-a—e.g., when bit line 115-a is designated as thepositive terminal. In some cases, memory cell 105-a may be accessed bysimultaneously applying a first voltage having a first polarity to wordline 110-a and a second voltage having a second polarity to bit line115-a, where the voltages applied to the word line 110-a and bit line115-a are additive across the memory cell 105-a, and the resultingvoltage applied to target memory cell 105-a is cell access voltage 240.

FIG. 2B illustrates exemplary diagrams for the operation of SBM inaccordance with various aspects of the present disclosure. The exemplarydiagrams include first voltage plot 250, second voltage plot 251, thirdvoltage plot 252, and fourth voltage plot 253, which may depict aspectsof a memory operation performed to access a memory cell, such as memorycell 105-a, as discussed with reference to FIG. 2A. The exemplarydiagrams also include a table 254 mapping voltage sequences tonon-binary logic values.

Each of first voltage plot 250, second voltage plot 251, third voltageplot 252, and fourth voltage plot 253 include a voltage axis and a timeaxis and may depict the application of one or more voltages across amemory cell during a write operation.

First voltage plot 250 depicts aspects of a write operation for a firstlogic state (e.g., “00”). In some examples, a memory device selects amemory cell, such as memory cell 105-a, for a write operation. Asdiscussed above, memory cell 105-a may include first memory element 210having a first resistivity and being in a first phase (e.g., anamorphous phase) and second memory element 220 having a secondresistivity and being in a second phase (e.g., an amorphous orcrystalline phase), where the first and second phase may be the same ordifferent.

After selecting memory cell 105-a, the memory device may (e.g., via abiasing component) apply first voltage 255 across memory cell 105-a.First voltage 255 may be a positive voltage and may be classified as ahigh voltage/current pulse (relative to other voltages such as thirdvoltage 265 and sixth voltage 280). In some examples, the positivevoltage may be applied across memory cell 105-a by applying firstvoltage 255 to bit line 115-a while word line 110-a is maintained at avirtual ground. In some cases, the magnitude and duration of firstvoltage 255 is configured to avoid heating either first memory element210 or second memory element 220 to a respective melting temperature. Insome examples, the magnitude and duration of first voltage 255 isconfigured to induce movement of ions in first memory element 210 andsecond memory element 220 within a minimum timing of a memory operation.

In response to the application of first voltage 255, ions within firstmemory element 210 may move toward a top of first memory element 210 andions within second memory element 220 may move toward a top of secondmemory element 220. When the ions are located near a top of first memoryelement 210, a resistivity (e.g., resistance or voltage threshold) offirst memory element 210 may be higher than when the ions are locatednear the bottom of first memory element 210. Similarly, when the ionsare located near a top of second memory element 220, a resistivity ofsecond memory element 220 may be higher than when the ions are locatednear the bottom of second memory element 220.

Accordingly, memory cell 105-a may be in a high resistivity state afterthe application of first voltage 255 based on the combination of firstmemory element 210 being in a high resistivity state and second memoryelement 220 being in a high resistivity states. Also, first memoryelement 210 may remain in the first phase and second memory element 220may remain in the second phase during and after first voltage 255 isapplied based on the magnitude and duration of first voltage 255 notheating either of first memory element 210 or second memory element 220to a respective melting temperature.

The logic value stored by memory cell 105-a (e.g., “00”) may bedetermined based on the high resistivity state of memory cell 105-a. Forexample, the high resistivity state may correspond to non-binary logicvalue 00. In some cases, second memory element 220 may be used to storea first bit of the non-binary logic value (e.g., the MSB of the logicvalue) and first memory element 210 may be used to store a second bit ofthe non-binary logic value (e.g., the LSB of the logic value). That is,second memory element 220 may store a logic 0 based on being in a higherresistivity state and first memory element 210 may store a logic 0 basedon being in a higher resistivity state. And together second memoryelement 220 and first memory element 210 may store logic value 00. Thisway, first voltage sequence 282 may program a logic value 00 into memorycell 105-a by applying a positive, high intensity electric pulse tomemory cell 105-a, as depicted by table 254.

Second voltage plot 251 depicts aspects of a write operation for asecond logic state (e.g., “01”). In some examples, a memory deviceselects a memory cell, such as memory cell 105-a, for a write operation.After selecting memory cell 105-a, the memory device may (e.g., via abiasing component) apply second voltage 260 across memory cell 105-a.Second voltage 260 may have a positive polarity and may be classified asa high current pulse (relative to other voltages such as third voltage265 and sixth voltage 280). As discussed above, in response to theapplication of second voltage 260, ions within first memory element 210and second memory element 220 may move toward a top of first memoryelement 210 and second memory element 220, and memory cell 105-a may bein a high resistivity state after second voltage 260 is applied.

The memory device may then apply third voltage 265 across memory cell105-a. Third voltage 265 may have a negative polarity and may beclassified as a low current pulse (relative to other voltages such asfirst voltage 255, second voltage 260, fourth voltage 270, or fifthvoltage 275). In some cases, the magnitude and duration of third voltage265 is configured to induce movement of the ions within first memoryelement 210 but not the ions within second memory element 220. In somecases, the magnitude and duration of third voltage 265 is configured toinduce movement of the ions within first memory element 210 within acertain time constraint. In some cases, the duration of third voltage265 is shorter than the duration of second voltage 260.

In response to the application of third voltage 265, ions within firstmemory element 210 (which may be a narrow bandgap/low glass transitiontemperature material relative to second memory element 220) may movetoward a bottom of first memory element 210 while ions within secondmemory element 220 (which may be a wide bandgap/high glass transitiontemperature material relative to first memory element 210) may remain atthe top of second memory element 220. In some cases, the magnitude ofthird voltage 265 is configured so that ions in first memory element 210will move within first memory element 210, while ions in second memoryelement 220 will remain in their current location. As discussed above,when the ions are located near a bottom of first memory element 210, aresistivity of first memory element 210 may be lower than when the ionsare located near the top of first memory element 210.

Also, when the ions of second memory element 220 are near the top ofsecond memory element 220, memory element may have a higher resistancethan when the ions are near the bottom of second memory element 220.Accordingly, memory cell 105-a may be in a first intermediaryresistivity state after the application of third voltage 265 based onthe combination of first memory element 210 being in a lower resistivitystate and second memory element 220 being in a higher resistivity state.Also, first memory element 210 may remain in the first phase and secondmemory element 220 may remain in the second phase during and aftersecond voltage 260 and third voltage 265 are applied based on themagnitude and duration of second voltage 260 and third voltage 265 notheating either of first memory element 210 or second memory element 220to a respective melting temperature.

The logic value stored by memory cell 105-a (e.g., “01”) may bedetermined based on the first intermediary resistivity state of memorycell 105-a. For example, the first intermediary resistivity state maycorrespond to non-binary logic value 01. As discussed above, secondmemory element 220 may be used to store an MSB of the non-binary logicvalue and first memory element 210 may be used to store an LSB of thenon-binary logic value. That is, second memory element 220 may store alogic 0 based on being in a higher resistivity state and first memoryelement 210 may store a logic 1 based on being in a lower resistivitystate. And together second memory element 220 and first memory element210 may store logic value 01. This way, second voltage sequence 284 mayprogram a logic value 01 into memory cell 105-a by applying a positive,high intensity electric pulse to memory cell 105-a followed by anegative, low intensity electric pulse, as depicted by table 254.

Third voltage plot 252 depicts aspects of a write operation for a thirdlogic state (e.g., “11”). In some examples, a memory device selects amemory cell, such as memory cell 105-a, for a write operation. Afterselecting memory cell 105-a, the memory device may (e.g., via a biasingcomponent) apply fourth voltage 270 across memory cell 105-a. Fourthvoltage 270 may have a negative polarity and may be classified as a highcurrent pulse (relative to other voltages such as third voltage 265 andsixth voltage 280). In some examples, fourth voltage 270 may be appliedto word line 110-a while bit line 115-a may be maintained at a virtualground. In some cases, the magnitude and duration of fourth voltage 270is configured to avoid heating either first memory element 210 or secondmemory element 220 to a respective melting temperature. In someexamples, the magnitude and duration of first voltage 255 is configuredto induce movement of ions in first memory element 210 and second memoryelement 220 within a minimum timing of a memory operation.

In response to the application of fourth voltage 270, ions within firstmemory element 210 may move toward a bottom of first memory element 210and ions within second memory element 220 may move toward a bottom ofsecond memory element 220. As discussed above, first memory element 210and second memory element 220 may have a lower resistivity when the ionsare near a bottom of the respective memory elements. Accordingly, afterapplying fourth voltage 270, memory cell 105-a may be in a lowresistivity state. Also, first memory element 210 may remain in thefirst phase and second memory element 220 may remain in the second phaseduring and after fourth voltage 270 is applied based on the magnitudeand duration of fourth voltage 270 not heating either of first memoryelement 210 or second memory element 220 to a respective meltingtemperature.

The logic value stored by memory cell 105-a (e.g., “11”) may bedetermined based on the low resistivity state of memory cell 105-a. Forexample, the low resistivity state may correspond to non-binary logicvalue 11. As discussed above, second memory element 220 may be used tostore an MSB of the non-binary logic value and first memory element 210may be used to store an LSB of the non-binary logic value. That is,second memory element 220 may store a logic 1 based on being in a lowerresistivity state and first memory element 210 may store a logic 1 basedon being in a lower resistivity state. And together second memoryelement 220 and first memory element 210 may store logic value 11. Thisway, fourth voltage sequence 288 may program a logic value 11 intomemory cell 105-a by applying a negative, high intensity electric pulseto memory cell 105-a, as depicted by table 254.

Fourth voltage plot 253 depicts aspects of a write operation for afourth logic state (e.g., “10”). In some examples, a memory deviceselects a memory cell, such as memory cell 105-a, for a write operation.After selecting memory cell 105-a, the memory device may (e.g., via abiasing component) apply fifth voltage 275 across memory cell 105-a.Fifth voltage 275 may have a negative polarity and may be classified asa high current pulse (relative to other voltages such as third voltage265 and sixth voltage 280). As discussed above, in response to theapplication of fifth voltage 275, ions within first memory element 210and second memory element 220 may move toward a bottom of the respectivememory elements, and memory cell 105-a may be in a low resistivity stateafter fifth voltage 275 is applied.

The memory device may then apply sixth voltage 280 across memory cell105-a. Sixth voltage 280 may have a positive polarity and may beclassified as a low current pulse (relative to other voltages such asfirst voltage 255, second voltage 260, fourth voltage 270, or fifthvoltage 275). In some cases, the magnitude and duration of sixth voltage280 is configured to induce movement of the ions within first memoryelement 210 but not the ions within second memory element 220. In somecases, the magnitude and duration of sixth voltage 280 is configured toinduce movement of the ions within first memory element 210 within acertain time constraint. In some cases, the duration of sixth voltage280 is shorter than the duration of fifth voltage 275.

In response to the application of sixth voltage 280, ions within firstmemory element 210 (which may be a narrow bandgap/low glass transitiontemperature material) may move toward a top of first memory element 210while ions within second memory element 220 may remain at the bottom ofsecond memory element 220. In some cases, the magnitude of sixth voltage280 is configured so that ions in first memory element 210 will movewithin first memory element 210, while ions in second memory element 220will remain in their current location. As discussed above, when the ionsare located near a top of first memory element 210, a resistivity offirst memory element 210 may be higher than when the ions are locatednear the bottom of first memory element 210. Also, when the ions ofsecond memory element 220 are near the bottom of second memory element220, second memory element 220 may have a lower resistance than when theions are near the top of second memory element 220. Accordingly, memorycell 105-a may be in a second intermediary resistivity state after theapplication of sixth voltage 280 based on the combination of firstmemory element 210 being in a higher resistivity state and second memoryelement 220 being in a lower resistivity state. Also, first memoryelement 210 may remain in the first phase and second memory element 220may remain in the second phase during and after fifth voltage 275 andthird sixth 280 are applied based on the magnitude and duration of fifthvoltage 275 and sixth voltage 280 not heating either of first memoryelement 210 or second memory element 220 to a respective meltingtemperature.

The logic value stored by memory cell 105-a (e.g. “10”) may bedetermined based on the second intermediary resistivity state of memorycell 105-a. For example, the second intermediary resistivity state maycorrespond to non-binary logic value 10. The second intermediaryresistivity state may be distinct from the first intermediaryresistivity state. As discussed above, second memory element 220 may beused to store an MSB of the non-binary logic value and first memoryelement 210 may be used to store an LSB of the non-binary logic value.That is, second memory element 220 may store a logic 1 based on being ina higher resistivity state and first memory element 210 may store alogic 0 based on being in a lower resistivity state. And together secondmemory element 220 and first memory element 210 may store logic value10. This way, third voltage sequence 286 may program a logic value 10into memory cell 105-a by applying a negative, high intensity electricpulse to memory cell 105-a followed by a positive, low intensityelectric pulse, as depicted by table 254.

This way, memory cell 105-a may be programmable to have any of fourdistinct resistivity states, each of which may correspond to aparticular logic value (e.g., a non-binary logic value).

Although the above examples provide that the ions within a material movefrom one end of memory element to another in response to a voltage beingapplied, in some cases, the ions within a material stay at one end ofthe memory element in response to a voltage being applied—e.g., the ionswithin a memory element may remain at a bottom of the memory element inresponse to a negative voltage being applied across the memory element.

In some examples, a logic state stored at a memory cell, such as memorycell 105-a, may be read by applying a voltage with a negative polarityacross the memory cell—e.g., by applying a higher voltage to word line110-a than to bit line 115-a. Reading the memory cell by applying anegative voltage may enlarge the memory window. Reading the memory cellmay include applying a predetermined or increasingly negative voltageacross memory cell 105-a and observing the resulting current throughmemory cell 105-a—e.g., a first observed current may be associated witha first logic value, a second observed current may be associated with asecond logic value, a third observed current may be associated with athird logic value, and so on. For example, if a low current is observed,it may be determined that memory cell 105-a is in a low resistivitystate and is storing a corresponding logic value (e.g., “11”).

Although the above generally discusses writing non-binary logic statesto a memory cell including two memory elements, similar operations maybe applied to write non-binary logic states to a memory cell includingmore than two memory elements in any number. For example, for a memorycell with three segregable memory elements, three different voltages maybe used where a first voltage induces ion movement in all three memoryelements, a second voltage induces ion movement in two memory elements,and other voltages induce ion movement in some subset of memoryelements.

FIG. 3 illustrates an example of a memory array that supports SBM andexemplary voltage plots for the operation of SBM in accordance withvarious aspects of the present disclosure. Memory array 300 may be anexample of a memory array 200 as described with reference to FIGS. 1 and2A. Memory array 300 may include memory cell 105-b, word line 110-b, andbit line 115-b, which may be examples of a memory cell 105, word line110, and bit line 115, as described with reference to FIGS. 1 and 2 .

Memory cell 105-b may include top electrode 305, memory element 310, andbottom electrode 325. Memory element 310 may be an example of firstmemory element 210, as discuss with reference to FIG. 2 . In someexamples, memory element 310 performs the functions of both a selectioncomponent (e.g., prevent stray and leakage currents from flowing throughmemory cell 105-b) and a storage component (e.g., storing a logicvalue). Memory cell 105-b may be referred to as a self-selecting memorycell. In some examples, memory element 310 includes a segregablematerial. In some examples, memory element 310 includes a materialhaving a narrow bandgap and a low glass transition temperature (e.g.,SAG). In some examples, the material is in an amorphous phase. In someexamples, a segregation period for memory element 310 is shorter than aSET period for memory element 310.

Memory cell 105-b may store data in memory element 310 usingsegregation-based techniques. In some examples, first voltage plot 301and second voltage plot 302 depict the application of a voltage tomemory cell 105-b during an exemplary segregation-based accessoperation. In some cases, the voltage applied across memory cell 105-bis generated using both word line 110-b and bit line 115-b, as discussedwith reference to FIG. 2A.

First voltage plot 301 depicts aspects of a write operation for memorycell 105-b. In some examples, a memory device selects a memory cell,such as memory cell 105-b, for a write operation. As discussed above,memory cell 105-b includes memory element 310 having a first resistivitycorresponding to a first logic value (e.g., logic value 1) and being ina first phase (e.g., an amorphous phase). After selecting memory cell105-b, the memory device may apply first voltage 330 across memory cell105-b. First voltage 330 may be a positive voltage and may cause theions within memory element 310 to move to the top of memory element 310.Accordingly, a resistivity (e.g., a threshold voltage or resistance) ofmemory element 310 may increase to a first value. First voltage 330 maythen be removed from memory cell 105-b, and memory element 310 maymaintain the increased resistivity. In some cases, the increasedresistivity of memory element 310 corresponds to a second logic state(e.g., logic value 0). During and after the application of first voltage330, memory element 310 may remain in the amorphous phase.

In some examples, a magnitude and duration of first voltage 330 isconfigured so that the application of first voltage 330 will not heatmemory element 310 to its melting temperature. In some examples, amagnitude and duration of first voltage 330 is configured so that ionswithin memory element 310 move from one end of memory element 310 toanother—e.g., the duration and magnitude may be configured so that amajority of the ions within memory element 310 move to one end of memoryelement 310 within a certain period of time. In some cases, relativelyshort voltage pulses with relatively low magnitudes may be applied tomemory element 310.

Second voltage plot 302 depicts aspects of a write operation for memorycell 105-b. In some examples, a memory device selects a memory cell,such as memory cell 105-b, for a write operation. After selecting memorycell 105-b, second voltage 335 may be applied across memory cell 105-b.Second voltage 335 may be a negative voltage and may cause the ionswithin memory element 310 to move to the bottom of memory element 310.Accordingly, a resistivity of memory element 310 may decrease to asecond value. After second voltage 335 is removed from memory cell105-b, memory element 310 may maintain the decreased resistivity. Insome cases, the decreased resistivity of memory element 310 correspondsto a second logic state (e.g., logic value 1). A magnitude and durationof second voltage 335 may be similarly configured as the magnitude andduration for first voltage 330. This way, a resistivity of memoryelement 310 may be changed within a single material state by applyingvoltages of varying polarities.

Although the above examples provide that the ions within a material movefrom one end of memory element to another in response to a voltage beingapplied, in some cases, the ions within a material stay at one end ofthe memory element in response to a voltage being applied—e.g., the ionswithin a memory element may remain at a bottom of the memory element inresponse to a negative voltage being applied across the memory element.

FIG. 4 illustrates a block diagram 400 of a memory array 100-a thatsupports the operation of SBM in accordance with various embodiments ofthe present disclosure. Memory array 100-a may be referred to as anelectronic memory apparatus and includes memory controller 140-a andmemory cell 105-c, which may be examples of memory controller 140 andmemory cell 105 described with reference to FIGS. 1 to 3 . Memory array100-a may also include reference component 420 and latch 425. Thecomponents of memory array 100-a may be in electronic communication witheach other and may perform the functions described with reference toFIGS. 1-3 . In some cases, reference component 420, sense component125-a and latch 425 may be components of memory controller 140-a.

Memory controller 140-a may, in combination with other components applyvoltages throughout memory array 100-a, write data to memory cells105-c, read data from memory cells 105-c, and generally operate memoryarray 100-a as described in FIGS. 1-3 . Memory controller 140-a mayinclude biasing component 410 and timing component 415. In some cases,memory controller may include SBM component 445. Memory controller 140-amay be in electronic communication with word line 110-c, bit line 115-c,and sense component 125-a, which may be examples of a word line 110, bitline 115, and sense component 125, as described with reference to FIG. 1or 2 .

Memory controller 140-a may be configured to activate word line 110-c orbit line 115-c by applying voltages to those various nodes. For example,biasing component 410 may be configured to apply a voltage to operatememory cell 105-c to read or write memory cell 105-c as described above.In some cases, memory controller 140-a may include a row decoder, columndecoder, or both, as described with reference to FIG. 1 . This mayenable memory controller 140-a to access one or more memory cells 105.Biasing component 410 may also provide voltage potentials to referencecomponent 420 in order to generate a reference signal for sensecomponent 125-a. Additionally, biasing component 410 may provide voltagepotentials for the operation of sense component 125-a.

In some cases, memory controller 140-a may perform its operations usingtiming component 415. For example, timing component 415 may control thetiming of the various word line selections or plate biasing, includingtiming for switching and voltage application to perform the memoryfunctions, such as reading and writing, discussed herein. In some cases,timing component 415 may control the operations of biasing component410.

Reference component 420 may include various components to generate areference signal for sense component 125-a. Reference component 420 mayinclude circuitry configured to produce a reference signal.

Memory cell 105-c may include one or more memory elements that may beoperated using ion segregation, as discussed with reference to FIGS. 1-3. In some cases, memory cell 105-c includes a first memory elementhaving a wide bandgap and high glass transition temperature and a secondmemory element having a narrow bandgap and low glass transitiontemperature. In some examples, the first memory element may be in afirst state (e.g., an amorphous phase) and the second memory element maybe in a second state (e.g., a crystalline phase or an amorphous phase).In some cases, the first memory element and second memory element mayexperience ion segregation in response to different voltage—e.g., acertain voltage may cause ion segregation in one memory element but notanother.

Sense component 125-a may compare a signal from memory cell 105-c(through bit line 115-c) with a reference signal from referencecomponent 420. Upon determining the logic state, the sense component maythen store the output in latch 425, where it may be used in accordancewith the operations of an electronic device that memory array 100-a is apart.

SBM component 445 may provide information for operating a memory arrayusing SBM. For instance, SBM component 445 may provide voltage andcurrent information for operating memory cells 105-c. The information inSBM component 445 may be used by biasing component 410 and timingcomponent 415 when applying voltages to memory cell 105-c. In someexamples, SBM component 445 provides a magnitude and duration forapplying the first and second voltages to memory cell 105-c. Forexamples, SBM component 445 may store a magnitude and duration for thefirst voltage that avoids heating the memory element(s) within memorycell 105-c to a melting temperature. SBM component 445 may also store amagnitude and duration for the second voltage that causes ionsegregation in one memory element in memory cell 105-c but not anothermemory element.

In some examples, memory controller 140-a selects memory cell 105-c fora write operation, where memory cell 105-c includes a first materialhaving a first resistivity (e.g., first memory element 210) and a secondmaterial having a second resistivity (e.g., second memory element 220).Memory controller may also cause biasing component 410 to apply a firstvoltage to memory cell 105-c during the write operation. In some cases,after the first voltage is applied, the first material may have a thirdresistivity and the second material may have a fourth resistivity.Memory controller may also cause biasing component 410 to apply a secondvoltage to memory cell 105-c during the write operation. After thesecond voltage is applied, the first material may have the firstresistivity and the second material may have the fourth resistivity. Insome cases, the first material is in a first phase (e.g., an amorphousphase) and the second material is in a second phase (e.g., an amorphousor crystalline phase) before, during, and after the application of thefirst voltage and second voltages.

Sense component 125-a may determine a logic value of the memory cellbased at least in part on a resistivity of the first material and thesecond material—e.g., by passing a current through memory cell 105-c. Insome cases, a combination of the resistivity of the first material and aresistivity of the second material correspond to a logic value. Forinstance, a first combination of the first resistivity (e.g., high) andthe second resistivity (e.g., high) may corresponds to a first logicvalue (e.g., “00”), a second combination of the third resistivity (e.g.,low) and the fourth resistivity (e.g., low) corresponds to a secondlogic value (e.g., “11”), and a third combination of the firstresistivity (e.g., high) and the fourth resistivity (e.g., low)corresponds to a third logic value (e.g., “10”). And a fourthcombination of the third resistivity (e.g., low) and the secondresistivity (e.g., high) corresponds to a fourth logic value (e.g.,“01”).

In some cases, memory controller 140-a may identify a logic value tostore at memory cell 105-c and may determine a polarity for the firstvoltage applied by biasing component 410 and the second voltage appliedby biasing component 410 based on the logic value. For example, to writea logic “01,” memory controller 140-a may determine that the firstvoltage will have a positive polarity and that the second voltage willhave a negative polarity.

Memory controller 140-a may also identify a magnitude and duration forthe first and second voltages. In some cases, the magnitude and durationfor the first and second voltage is based on a glass transitiontemperature of the first material and a glass transition temperature ofthe second material—e.g., the magnitude and duration may be configuredso that an internal temperature of neither the first material or thesecond material reaches a respective melting temperature during or afterthe application of the first and second voltages. In some cases, amagnitude of the first voltage is higher than a magnitude of the secondvoltage. In some cases, the first voltage has a first polarity and thesecond voltage has a second polarity.

In some examples, memory controller 140-a selects memory cell 105-c fora first write operation. Memory cell 105-c may include a first material(e.g., first memory element 210 or memory element 310) in a first phase(e.g., an amorphous phase) and having a first resistivity—e.g., wherethe first resistivity of the first material corresponds to a first logicvalue stored by the first material. Memory controller 140-a may causebiasing component 410 to apply a first voltage to memory cell 105-cduring the write operation. In some cases, the first material may remainin the first phase during and after the application of the first voltageand may have a second resistivity that corresponds to a second logicvalue after the first voltage is applied.

In some cases, memory controller 140-a causes biasing component 410 toapply a second voltage to memory cell 105-c during a second writeoperation, where the first material may remain in the first phase duringand after the application of the second voltage, but may have the firstresistivity after the application of the second voltage. In some cases,the first resistivity is lower than the second resistivity. In somecases, the first voltage has a first polarity and the second voltage hasa second polarity. In some cases, ions within the first material movetoward an end of the first material in response to the first voltagebeing applied, and a value of the second resistivity is based on the ionmovement—e.g., if the ions move to a top of the first material, thesecond resistivity may be higher than if the ions were located a bottomof the first material.

In some cases, memory cell 105-c includes a second material (e.g.,second memory element 220) in a second phase (e.g., an amorphous orcrystalline phase) and having a third resistivity that corresponds to athird logic value stored by the second material. In some cases, acombination of the second resistivity and the third resistivitycorresponds to a non-binary logic value stored by the memory cell. Thefirst voltage may be applied to both the first and second materials ofmemory cell 105-c. In some cases, the second material has a fourthresistivity after the first voltage is applied, but remains in thesecond phase during and after the application of the first voltage. Insome cases, the second material in the second phase has the thirdresistivity after the first voltage is applied—e.g., if the ions withinthe second material are located at a particular end of the secondmaterial before the first voltage is applied. In some examples, thefirst material is programmable with a first current and the secondmaterial is programmable with a second current having a greatermagnitude and/or shorter duration than the first current.

Memory controller 140-a may cause biasing component 410 to apply asecond voltage to memory cell 105-c during the first write operation.After the second voltage being applied, the first material may remain inthe first phase and have the first resistivity and the second materialmay remain in the second phase and have the fourth resistivity.

FIG. 5 illustrates a system, including a memory array, that supports theoperation of SBM in accordance with various embodiments of the presentdisclosure. System 500 may include a device 505, which may be or includea printed circuit board to connect or physically support variouscomponents. Device 505 includes a memory array 100-b, which may be anexample of memory array 100 described with reference to FIGS. 1 and 4 .Memory array 100-b may contain memory controller 140-b and memorycell(s) 105-d, which may be examples of memory controller 140 describedwith reference to FIGS. 1 and 4 and memory cells 105 described withreference to FIG. 1-4 . Device 505 may also include a processor 510,basic input/output system (BIOS) component 515, peripheral component(s)520, and input/output control component 525. The components of device505 may be in electronic communication with one another through bus 530.

Processor 510 may be configured to operate memory array 100-b throughmemory controller 140-b. In some cases, processor 510 may perform thefunctions of memory controller 140 described with reference to FIGS. 1and 4 . In other cases, memory controller 140-b may be integrated intoprocessor 510. Processor 510 may be a general-purpose processor, adigital signal processor (DSP), an application-specific integratedcircuit (ASIC), a field-programmable gate array (FPGA) or otherprogrammable logic device, discrete gate or transistor logic, discretehardware components, or it may be a combination of these types ofcomponents, and processor 510 may perform various functions describedherein, including the operation of SBM. Processor 510 may, for example,be configured to execute computer-readable instructions stored in memoryarray 100-b to cause device 505 perform various functions or tasks.

BIOS component 515 may be a software component that includes a BIOSoperated as firmware, which may initialize and run various hardwarecomponents of system 500. BIOS component 515 may also manage data flowbetween processor 510 and the various components, e.g., peripheralcomponents 520, input/output control component 525, etc. BIOS component515 may include a program or software stored in read-only memory (ROM),flash memory, or any other non-volatile memory.

Peripheral component(s) 520 may be any input or output device, or aninterface for such devices, that is integrated into device 505. Examplesmay include disk controllers, sound controller, graphics controller,Ethernet controller, modem, universal serial bus (USB) controller, aserial or parallel port, or peripheral card slots, such as peripheralcomponent interconnect (PCI) or accelerated graphics port (AGP) slots.

Input/output control component 525 may manage data communication betweenprocessor 510 and peripheral component(s) 520, input devices 535, oroutput devices 545. Input/output control component 525 may also manageperipherals not integrated into device 505. In some cases, input/outputcontrol component 525 may represent a physical connection or port to theexternal peripheral.

Input device 535 may represent a device or signal external to device 505that provides input to device 505 or its components. This may include auser interface or interface with or between other devices. In somecases, input device 535 may be a peripheral that interfaces with device505 via peripheral component(s) 520 or may be managed by input/outputcontrol component 525.

Output device 545 may represent a device or signal external to device505 configured to receive output from device 505 or any of itscomponents. Examples of output device 545 may include a display, audiospeakers, a printing device, another processor or printed circuit board,etc. In some cases, output device 545 may be a peripheral thatinterfaces with device 505 via peripheral component(s) 520 or may bemanaged by input/output control component 525.

The components of memory controller 140-b, device 505, and memory array100-b may be made up of circuitry designed to carry out their functions.This may include various circuit elements, for example, conductivelines, transistors, capacitors, inductors, resistors, amplifiers, orother active or inactive elements, configured to carry out the functionsdescribed herein.

FIG. 6 illustrates a flowchart of a method or methods for the operationof SBM in accordance with various embodiments of the present disclosure.Method 600 may illustrate aspects of storing a non-binary logic value ata memory cell containing multiple segregable materials by individuallyprogramming a resistance of the different materials single using ionsegregation principles.

At block 605, the method may include selecting a memory cell for a writeoperation, the memory cell comprising a first material having a firstresistivity and a second material having a second resistivity, asdescribed with reference to FIGS. 1-3 . In certain examples, theoperations of block 605 may be performed or facilitated by a memorycontroller, as described with reference to FIGS. 1, 4, and 5 .

At block 610, the method may include applying a first voltage to thememory cell during the write operation, the first material having athird resistivity and the second material having a fourth resistivityafter applying the first voltage, as described with reference to FIGS.1-3 . In certain examples, the operations of block 610 may be performedor facilitated by a memory controller, as described with reference toFIGS. 1, 4, and 5 .

At block 615, the method may include applying a second voltage to thememory cell during the write operation, the first material having thefirst resistivity and the second material having the fourth resistivityafter applying the second voltage, wherein the first material is in afirst phase before the first voltage is applied and after the secondvoltage is applied and the second material is in a second phase beforethe first voltage is applied and after the second voltage is applied, asdescribed with reference to FIGS. 1-3 . In certain examples, theoperations of block 615 may be performed or facilitated by a memorycontroller, as described with reference to FIGS. 1, 4, and 5 .

In some examples, an apparatus as described herein may perform a methodor methods, such as the method 600. The apparatus may include features,means, or instructions (e.g., a non-transitory computer-readable mediumstoring instructions executable by a processor) for selecting a memorycell for a write operation, the memory cell including a first materialhaving a first resistivity and a second material having a secondresistivity, applying a first voltage to the memory cell during thewrite operation, the first material having a third resistivity and thesecond material having a fourth resistivity after applying the firstvoltage, and applying a second voltage to the memory cell during thewrite operation, the first material having the first resistivity and thesecond material having the fourth resistivity after applying the secondvoltage, where the first material is in a first phase before the firstvoltage is applied and after the second voltage is applied and thesecond material is in a second phase before the first voltage is appliedand after the second voltage is applied.

Some examples of the method, apparatuses, and non-transitorycomputer-readable medium described herein may further includeoperations, features, means, or instructions for determining a logicvalue of the memory cell based on a resistivity of the first materialand a resistivity of the second material.

In some examples of the method, apparatuses, and non-transitorycomputer-readable medium described herein, a first combination of thefirst resistivity and the second resistivity corresponds to a firstlogic value, a second combination of the third resistivity and thefourth resistivity corresponds to a second logic value, and a thirdcombination of the first resistivity and the fourth resistivitycorresponds to a third logic value.

Some examples of the method, apparatuses, and non-transitorycomputer-readable medium described herein may further includeoperations, features, means, or instructions for identifying a logicvalue to store at the memory cell via the write operation anddetermining a polarity for the first voltage and a polarity for thesecond voltage based on the logic value.

In some examples of the method, apparatuses, and non-transitorycomputer-readable medium described herein, the first voltage may bebased on a first glass transition temperature of the first material andthe second voltage may be based on a second glass transition temperatureof the second material, where the second glass transition temperaturegreater than the first glass transition temperature.

Some examples of the method, apparatuses, and non-transitorycomputer-readable medium described herein may further includeoperations, features, means, or instructions for selecting the memorycell for a read operation, applying a third voltage to the memory cellduring the read operation, and determining a logic value stored by thememory cell based on applying the third voltage.

In some examples of the method, apparatuses, and non-transitorycomputer-readable medium described herein, a resistivity of the firstmaterial corresponds to a first threshold voltage and a resistivity ofthe second material corresponds to a second threshold voltage.

In some examples of the method, apparatuses, and non-transitorycomputer-readable medium described herein, a magnitude of the firstvoltage may be higher than a magnitude of the second voltage.

In some examples of the method, apparatuses, and non-transitorycomputer-readable medium described herein, the first voltage may have afirst polarity and the second voltage may have a second polarity.

FIG. 7 illustrates a flowchart of a method or methods for the operationof SBM in accordance with various embodiments of the present disclosure.Method 700 may illustrate aspects of programming a resistance of amemory cell containing a single segregable material by causing ionmovement within the material.

At block 705, the method may include selecting a memory cell for a writeoperation, the memory cell comprising a first material in a first phasehaving a first resistivity that corresponds to a first logic valuestored by the first material, as described with reference to FIGS. 1-3 .In certain examples, the operations of block 705 may be performed orfacilitated by a memory controller, as described with reference to FIGS.1, 4, and 5 .

At block 710, the method may include applying a first voltage to thememory cell during the write operation, wherein the first materialremains in the first phase and has a second resistivity that correspondsto a second logic value stored by the first material after applying thefirst voltage, as described with reference to FIGS. 1-3 . In certainexamples, the operations of block 710 may be performed or facilitated bya memory controller, as described with reference to FIGS. 1, 4, and 5 .

In some examples, an apparatus as described herein may perform a methodor methods, such as the method 600. The apparatus may include features,means, or instructions (e.g., a non-transitory computer-readable mediumstoring instructions executable by a processor) for selecting a memorycell for a write operation, the memory cell including a first materialin a first phase having a first resistivity that corresponds to a firstlogic value stored by the first material and applying a first voltage tothe memory cell during the write operation, where the first materialremains in the first phase and has a second resistivity that correspondsto a second logic value stored by the first material after applying thefirst voltage.

Some examples of the method, apparatuses, and non-transitorycomputer-readable medium described herein may further includeoperations, features, means, or instructions for applying a secondvoltage to the memory cell during a second write operation, the firstmaterial remaining in the first phase and having the first resistivityafter the second voltage may be applied.

In some examples of the method, apparatuses, and non-transitorycomputer-readable medium described herein, the first voltage may have afirst polarity and the second voltage may have a second polarity, andwhere the first resistivity may be lower than the second resistivity.

In some examples of the method, apparatuses, and non-transitorycomputer-readable medium described herein, the memory cell includes asecond material in a second phase and having a third resistivity thatcorresponds to a third logic value stored by the second material, andwhere a combination of the second resistivity and the third resistivitycorresponds to a non-binary logic value stored by the memory cell.

Some examples of the method, apparatuses, and non-transitorycomputer-readable medium described herein may further includeoperations, features, means, or instructions for applying a secondvoltage to the memory cell during the write operation, the firstmaterial remaining in the first phase and having the first resistivityand the second material remaining in the second phase and having thefourth resistivity after the second voltage may be applied.

In some examples of the method, apparatuses, and non-transitorycomputer-readable medium described herein, the second material in thesecond phase may have the third resistivity after the first voltage maybe applied.

In some examples of the method, apparatuses, and non-transitorycomputer-readable medium described herein, the first phase may be anamorphous phase and the second phase may be a crystalline phase. Inother examples of the method, apparatuses, and non-transitorycomputer-readable medium described herein, the first phase may be anamorphous phase and the second phase may be an amorphous phase.

In some examples of the method, apparatuses, and non-transitorycomputer-readable medium described herein, the first material may beprogrammable with a first current and the second material may beprogrammable with a second current, the first current being lower thanthe second current.

In some examples of the method, apparatuses, and non-transitorycomputer-readable medium described herein, ions of the first materialmove toward an end of the first material after the first voltage may beapplied, and where a value of the second resistivity may be based on theion movement.

It should be noted that the methods described above describe possibleimplementations, and that the operations and the steps may be rearrangedor otherwise modified and that other implementations are possible.Furthermore, embodiments from two or more of the methods may becombined.

In some examples, an apparatus or device may perform aspects of thefunctions described herein using general, or special-purpose hardware.The apparatus or device may include a memory cell including a firstmaterial in a first phase and a second material in a second phase; afirst access line in electronic communication with the memory cell; anda second access line in electronic communication with the memory cell.In some cases, a resistivity of the first material is programmablewithin the first phase using an amount of current above a firstthreshold and a resistivity of the second material is programmablewithin the second phase using an amount of current above a secondthreshold.

In some examples of the apparatus or device, the memory cell includes amemory element comprising the first material and the second material,where the first material is configured to store a first bit of a logicvalue and the second material configured to store a second bit of thelogic value.

In some examples of the apparatus or device, a first bandgap energy ofthe first material is lower than a second bandgap energy of the secondmaterial, a resistivity of the first material is programmable within thefirst phase, and a resistivity of the second material is programmablewithin the second phase with an amount of current below a thirdthreshold. The third threshold may be based at least in part on thefirst bandgap and the second bandgap.

In some examples of the apparatus or device, a first glass transitiontemperature of the first material is between 350 Kelvin and 400 Kelvinand a second glass transition temperature of the second material isbetween 400 Kelvin and 450 Kelvin

In some examples of the apparatus or device, a first resistivity of thefirst material corresponds to a first segregated state of ions withinthe first material and a second resistivity of the first materialcorresponds to a second segregated state of ions within the firstmaterial. And a third resistivity of the second material corresponds toa third segregated state of ions within the second material and a fourthresistivity of the second material corresponds to a fourth segregatedstate of ions within the second material.

In some examples of the apparatus or device, the first phase is anamorphous phase and the second phase is a crystalline phase and thefirst threshold is lower than the second threshold. In other examples ofthe apparatus or device, the first phase is an amorphous phase and thesecond phase is an amorphous phase and the first threshold is lower thanthe second threshold.

In some examples of the apparatus or device, a resistivity of the firstmaterial corresponds to a first threshold voltage of the first materialand a resistivity of the second material corresponds to a secondthreshold voltage of the second material.

As used herein, the term “virtual ground” refers to a node of anelectrical circuit that is held at a voltage of approximately zero volts(0V) but that is not directly connected with ground. Accordingly, thevoltage of a virtual ground may temporarily fluctuate and return toapproximately 0V at steady state. A virtual ground may be implementedusing various electronic circuit elements, such as a voltage dividerconsisting of operational amplifiers and resistors. Otherimplementations are also possible. “Virtual grounding” or “virtuallygrounded” means connected to approximately 0V.

The term “electronic communication” and “coupled” refer to arelationship between components that support electron flow between thecomponents. This may include a direct connection between components ormay include intermediate components. Components in electroniccommunication or coupled to one another may be actively exchangingelectrons or signals (e.g., in an energized circuit) or may not beactively exchanging electrons or signals (e.g., in a de-energizedcircuit) but may be configured and operable to exchange electrons orsignals upon a circuit being energized. By way of example, twocomponents physically connected via a switch (e.g., a transistor) are inelectronic communication or may be coupled regardless of the state ofthe switch (e.g., open or closed).

The term “isolated” or “electrically isolated” refers to a relationshipbetween components in which electrons are not presently capable offlowing between them; components are isolated from each other if thereis an open circuit between them. For example, two components physicallyconnected by a switch may be isolated from each other when the switch isopen.

The term “layer” used herein refers to a stratum or sheet of ageometrical structure. Each layer may have three dimensions (e.g.,height, width, and depth) and may cover some or all of a surface. Forexample, a layer may be a three-dimensional structure where twodimensions are greater than a third. Layers may include differentelements, components, and/or materials. In some cases, one layer may becomposed of two or more sublayers. In some of the appended figures, twodimensions of a three-dimensional layer are depicted for purposes ofillustration. Those skilled in the art will, however, recognize that thelayers are three-dimensional in nature.

As used herein, the term “substantially” means that the modifiedcharacteristic (e.g., a verb or adjective modified by the termsubstantially) need not be absolute but is close enough so as to achievethe advantages of the characteristic.

As used herein, the term “electrode” may refer to an electricalconductor, and in some cases, may be employed as an electrical contactto a memory cell or other component of a memory array. An electrode mayinclude a trace, wire, conductive line, conductive layer, or the likethat provides a conductive path between elements or components of memoryarray 100.

Chalcogenide materials may be materials or alloys that include at leastone of the elements S, Se, and Te. Phase change materials discussedherein may be chalcogenide materials. Chalcogenide materials may includealloys of S, Se, Te, Ge, As, Al, Sb, Au, indium (In), gallium (Ga), tin(Sn), bismuth (Bi), palladium (Pd), cobalt (Co), oxygen (O), silver(Ag), nickel (Ni), platinum (Pt). Example chalcogenide materials andalloys may include, but are not limited to, Ge—Te, In—Se, Sb—Te, Ga—Sb,In—Sb, As—Te, Al—Te, Ge—Sb—Te, Te—Ge—As, In—Sb—Te, Te—Sn—Se, Ge—Se—Ga,Bi—Se—Sb, Ga—Se—Te, Sn—Sb—Te, In—Sb—Ge, Te—Ge—Sb—S, Te—Ge—Sn—O,Te—Ge—Sn—Au, Pd—Te—Ge—Sn, In—Se—Ti—Co, Ge—Sb—Te—Pd, Ge—Sb—Te—Co,Sb—Te—Bi—Se, Ag—In—Sb—Te, Ge—Sb—Se—Te, Ge—Sn—Sb—Te, Ge—Te—Sn—Ni,Ge—Te—Sn—Pd, or Ge—Te—Sn—Pt. The hyphenated chemical compositionnotation, as used herein, indicates the elements included in aparticular compound or alloy and is intended to represent allstoichiometries involving the indicated elements. For example, Ge—Te mayinclude Ge_(x)Te_(y), where x and y may be any positive integer. Otherexamples of variable resistance materials may include binary metal oxidematerials or mixed valence oxide including two or more metals, e.g.,transition metals, alkaline earth metals, and/or rare earth metals.Embodiments are not limited to a particular variable resistance materialor materials associated with the memory elements of the memory cells.For example, other examples of variable resistance materials can be usedto form memory elements and may include chalcogenide materials, colossalmagnetoresistive materials, or polymer-based materials, among others.

The term “photolithography,” as used herein, may refer to the process ofpatterning using photoresist materials and exposing such materials usingelectromagnetic radiation. For example, a photoresist material may beformed on a base material by, for example, spin-coating the photoresiston the base material. A pattern may be created in the photoresist byexposing the photoresist to radiation. The pattern may be defined by,for example, a photo mask that spatially delineates where the radiationexposes the photoresist. Exposed photoresist areas may then be removed,for example, by chemical treatment, leaving behind the desired pattern.In some cases, the exposed regions may remain and the unexposed regionsmay be removed.

As used herein, the term “short,” “shorted, or “shorting” refers to arelationship between components in which a conductive path isestablished between the components via the activation of a singleintermediary component between the two components in question. Forexample, a first component shorted to a second component may exchangeelectrons with the second component when a switch between the twocomponents is closed. Thus, shorting may be a dynamic operation thatenables the flow of charge between components (or lines) that are inelectronic communication.

The devices discussed herein, including memory array 100, may be formedon a semiconductor substrate, such as silicon, germanium,silicon-germanium alloy, gallium arsenide, gallium nitride, etc. In somecases, the substrate is a semiconductor wafer. In other cases, thesubstrate may be a silicon-on-insulator (SOI) substrate, such assilicon-on-glass (SOG) or silicon-on-sapphire (SOP), or epitaxial layersof semiconductor materials on another substrate. The conductivity of thesubstrate, or sub-regions of the substrate, may be controlled throughdoping using various chemical species including, but not limited to,phosphorous, boron, or arsenic. Doping may be performed during theinitial formation or growth of the substrate, by ion-implantation, or byany other doping means.

A transistor or transistors discussed herein may represent afield-effect transistor (FET) and comprise a three terminal deviceincluding a source, drain, and gate. The terminals may be connected toother electronic elements through conductive materials, e.g., metals.The source and drain may be conductive and may comprise a heavily-doped,e.g., degenerate, semiconductor region. The source and drain may beseparated by a lightly-doped semiconductor region or channel. If thechannel is n-type (e.g., majority carriers are electrons), then the FETmay be referred to as a n-type FET. If the channel is p-type (e.g.,majority carriers are holes), then the FET may be referred to as ap-type FET. The channel may be capped by an insulating gate oxide. Thechannel conductivity may be controlled by applying a voltage to thegate. For example, applying a positive voltage or negative voltage to ann-type FET or a p-type FET, respectively, may result in the channelbecoming conductive. A transistor may be “on” or “activated” when avoltage greater than or equal to the transistor's threshold voltage isapplied to the transistor gate. The transistor may be “off” or“deactivated” when a voltage less than the transistor's thresholdvoltage is applied to the transistor gate.

The detailed description set forth above in connection with the appendeddrawings describes examples and does not represent the only examplesthat may be implemented or that are within the scope of the claims. Theterms “example” and “exemplary,” when used in this description, mean“serving as an example, instance, or illustration,” and not “preferred”or “advantageous over other examples.” The detailed description includesspecific details for the purpose of providing an understanding of thedescribed techniques. These techniques, however, may be practicedwithout these specific details. In some instances, well-known structuresand apparatuses are shown in block diagram form in order to avoidobscuring the concepts of the described examples.

Information and signals may be represented using any of a variety ofdifferent technologies and techniques. For example, data, instructions,commands, information, signals, bits, symbols, and chips that may bereferenced throughout the above description may be represented byvoltages, currents, electromagnetic waves, magnetic fields or particles,optical fields or particles, or any combination thereof.

The various illustrative blocks and components described in connectionwith the disclosure herein may be implemented or performed with ageneral-purpose processor, a digital signal processor (DSP), an ASIC, anFPGA or other programmable logic device, discrete gate or transistorlogic, discrete hardware components, or any combination thereof designedto perform the functions described herein. A general-purpose processormay be a microprocessor, but in the alternative, the processor may beany conventional processor, controller, microcontroller, or statemachine. A processor may also be implemented as a combination ofcomputing devices, e.g., a combination of a DSP and a microprocessor,multiple microprocessors, one or more microprocessors in conjunctionwith a DSP core, or any other such configuration.

The functions described herein may be implemented in hardware, softwareexecuted by a processor, firmware, or any combination thereof. Ifimplemented in software executed by a processor, the functions may bestored on or transmitted over as one or more instructions or code on acomputer-readable medium. Other examples and implementations are withinthe scope and spirit of the disclosure and appended claims. For example,due to the nature of software, functions described above can beimplemented using software executed by a processor, hardware, firmware,hardwiring, or combinations of any of these. Features implementingfunctions may also be physically located at various positions, includingbeing distributed such that portions of functions are implemented atdifferent physical locations. As used herein, including in the claims,the term “and/or,” when used in a list of two or more items, means thatany one of the listed items can be employed by itself, or anycombination of two or more of the listed items can be employed. Forexample, if a composition is described as containing components A, B,and/or C, the composition can contain A alone; B alone; C alone; A and Bin combination; A and C in combination; B and C in combination; or A, B,and C in combination. Also, as used herein, including in the claims,“or” as used in a list of items (for example, a list of items prefacedby a phrase such as “at least one of” or “one or more of”) indicates adisjunctive list such that, for example, a list of “at least one of A,B, or C” means A or B or C or AB or AC or BC or ABC (i.e., A and B andC).

Computer-readable media includes both computer storage media andcommunication media including any medium that facilitates transfer of acomputer program from one place to another. A storage medium may be anyavailable medium that can be accessed by a general purpose or specialpurpose computer. By way of example, and not limitation,computer-readable media can comprise RAM, ROM, EEPROM, flash memory,CD-ROM or other optical disk storage, magnetic disk storage or othermagnetic storage devices, or any other medium that can be used to carryor store desired program code means in the form of instructions or datastructures and that can be accessed by a general-purpose orspecial-purpose computer, or a general-purpose or special-purposeprocessor. Also, any connection is properly termed a computer-readablemedium. For example, if the software is transmitted from a website,server, or other remote source using a coaxial cable, fiber optic cable,twisted pair, digital subscriber line (DSL), or wireless technologiessuch as infrared, radio, and microwave, then the coaxial cable, fiberoptic cable, twisted pair, DSL, or wireless technologies such asinfrared, radio, and microwave are included in the definition of medium.Disk and disc, as used herein, include compact disc (CD), laser disc,optical disc, digital versatile disc (DVD), floppy disk and Blu-ray discwhere disks usually reproduce data magnetically, while discs reproducedata optically with lasers. Combinations of the above are also includedwithin the scope of computer-readable media.

The previous description of the disclosure is provided to enable aperson skilled in the art to make or use the disclosure. Variousmodifications to the disclosure will be readily apparent to thoseskilled in the art, and the generic principles defined herein may beapplied to other variations without departing from the scope of thedisclosure. Thus, the disclosure is not to be limited to the examplesand designs described herein but is to be accorded the broadest scopeconsistent with the principles and novel features disclosed herein.

What is claimed is:
 1. An apparatus, comprising: a memory cellcomprising a first material in a first phase and a second material in asecond phase, wherein: a resistivity of the first material isprogrammable within the first phase using an amount of current above afirst threshold; and a resistivity of the second material isprogrammable within the second phase using an amount of current above asecond threshold; a first access line in electronic communication withthe memory cell; and a second access line in electronic communicationwith the memory cell.
 2. The apparatus of claim 1, wherein the memorycell comprises a memory element comprising the first material and thesecond material, the first material configured to store a first bit of alogic value and the second material configured to store a second bit ofthe logic value.
 3. The apparatus of claim 1, wherein a first bandgap ofthe first material is lower than a second bandgap of the secondmaterial, and wherein a resistivity of the first material isprogrammable within the first phase and a resistivity of the secondmaterial is programmable within the second phase with an amount ofcurrent below a third threshold, the third threshold being based atleast in part on the first bandgap and the second bandgap.
 4. Theapparatus of claim 1, wherein a first glass transition temperature ofthe first material is between 350 Kelvin and 400 Kelvin and a secondglass transition temperature of the second material is between 400Kelvin and 450 Kelvin.
 5. The apparatus of claim 1, wherein the firstphase is an amorphous state and the second phase is a crystalline state,and wherein the first threshold is lower than the second threshold. 6.The apparatus of claim 1, wherein a resistivity of the first materialcorresponds to a first threshold voltage of the first material and aresistivity of the second material corresponds to a second thresholdvoltage of the second material.
 7. An apparatus, comprising: a memorycell comprising a first material in a first phase and a second materialin a second phase, wherein: based at least in part on a first voltagebeing applied to the memory cell, the first material and the secondmaterial are configured to switch from respectively having a firstresistivity and a second resistivity to respectively having a thirdresistivity and a fourth resistivity while respectively remaining in thefirst phase and the second phase; and based at least in part on a secondvoltage being applied to the memory cell, the first material isconfigured to switch from having the third resistivity to having thefirst resistivity while remaining in the first phase and while thesecond material continues to have the fourth resistivity and be in thesecond phase.
 8. The apparatus of claim 7, wherein the memory cell isconfigured to store a logic value based at least in part on aresistivity of the first material and a resistivity of the secondmaterial.
 9. The apparatus of claim 8, wherein a first combination ofthe first resistivity and the second resistivity corresponds to a firstlogic value, a second combination of the third resistivity and thefourth resistivity corresponds to a second logic value, and a thirdcombination of the first resistivity and the fourth resistivitycorresponds to a third logic value.
 10. The apparatus of claim 7,wherein the memory cell is configured to store a logic value based atleast in part on a polarity of the first voltage and a polarity of thesecond voltage.
 11. The apparatus of claim 7, wherein the secondmaterial has a higher glass transition temperature than the firstmaterial.
 12. The apparatus of claim 7, wherein a resistivity of thefirst material corresponds to a threshold voltage of the first materialand a resistivity of the second material corresponds to a thresholdvoltage of the second material.
 13. The apparatus of claim 7, wherein amagnitude of the first voltage is higher than a magnitude of the secondvoltage.
 14. The apparatus of claim 7, wherein the first voltage has afirst polarity and the second voltage has a second polarity.
 15. Anapparatus, comprising: a memory cell comprising: a first material,wherein: in response to a first voltage being applied to the memorycell, the first material is configured to switch from having a firstresistivity corresponding to a first logic value to having a secondresistivity corresponding to a second logic value, the switching of thefirst material from having the first resistivity to having the secondresistivity based at least in part on at least a portion of ions withinthe first material moving toward an end of the first material while thefirst material remains in a first phase; and a second material in asecond phase, wherein the memory cell is configured to store a logicvalue based at least in part on a combination of a resistivity of thefirst material and a resistivity of the second material, and wherein thefirst phase is an amorphous phase and the second phase is a crystallinephase.
 16. The apparatus of claim 15, wherein the first voltage has afirst polarity and a second voltage has a second polarity, and whereinthe first resistivity is lower than the second resistivity.
 17. Theapparatus of claim 15, wherein the logic value is a non-binary logicvalue.
 18. The apparatus of claim 17, wherein: in response to the firstvoltage being applied to the memory cell, the second material isconfigured to remain in the second phase and switch from having a thirdresistivity to having a fourth resistivity; and in response to a secondvoltage being applied to the memory cell, the first material isconfigured to remain in the first phase and switch from having thesecond resistivity to having the first resistivity, and the secondmaterial is configured to remain in the second phase and continue havingthe fourth resistivity.